1. Field of the Invention
This invention relates generally to a semiconductor memory device, and, more specifically, to providing a refresh oscillator scheme that is capable of compensating for external factors, such as voltage, temperature, and process.
2. Description of the Related Art
Modern integrated circuit devices are comprised of millions of semiconductor devices, e.g., transistors, formed above a semiconductor substrate, such as silicon. These devices are very densely packed, i.e., there is little space between them. Similarly, densely packed electrically conducting lines may also be formed in the semiconductor substrate. By forming selected electrical connections between selected semiconductor devices and selected conducting lines, circuits capable of performing complex functions may be created. For example, bits of data may be stored by providing electrical current to a plurality of bit lines and an orthogonal plurality of wordlines that may be electrically coupled to one or more capacitors in a semiconductor memory.
The semiconductor memory may be a dynamic random access memory, a flash memory, and the like. The semiconductor memory typically comprises an array of memory cells, address decoding circuitry for selecting one, or a group, of the memory cells for reading or writing data, sensing circuitry for detecting the digital state of the selected memory cell or memory cells, and input/output lines to receive the sensed data and convey that information for eventual output from the semiconductor memory. In many cases, the array of memory cells will be sub-divided into several sub-arrays, or subsets, of the complete collection of memory cells. For example, a semiconductor memory having 16 megabits (224 bits) of storage capacity, may be divided into 64 sub-arrays, each having 256K (218) memory cells.
Many of today's memory devices, such as Synchronous RAM (SDRAM), double data rate RAM devices (DDR RAM), and the like, require intricate timing schemes for proper operation. Important timing functions include proper implementation of refresh cycles that are used to maintain the integrity of the data stored in memory. Many times, memory devices and other electronic devices may utilize refresh-type circuitry that refreshes a memory cell to maintain memory integrity. However, the refresh rates and operation of maintaining memory integrity may be affected by external factors such as temperature variations, operating voltage, the type of processes used to manufacture memory devices, etc.
Designers of electronic devices, e.g., memory devices, have to consider a variety of voltage ranges and temperature ranges when designing circuits that form the devices. For example, the refresh rates may be predetermined by the upper or lower boundary thresholds of temperature ranges that are used to qualify the operating range of the devices. Often, these temperature ranges include an upper boundary of 85° C. Therefore, refresh cycles that are designed to operate at upper temperature boundaries, e.g., 85° C., may not provide for an efficient design for normal use in lower temperature ranges. Therefore, excessive amounts of power may be consumed when refresh rates are implemented to operate in the extreme range of temperature and operating voltage tolerances.
Generally, as the temperature rises, the memory cells in a memory device may experience a loss of ability to hold a charge because current leakage may be induced by higher temperatures. Often, refresh rates are implemented to accommodate higher temperatures, however at lower temperatures, the same higher refresh rates are used; thereby increasing standby current unnecessarily and causing excessive power consumption.
Power consumption is an important factor in electronic devices. It is desirable to reduce power consumption when implementing certain applications. In particular, wireless and battery operated equipment require lower power consumption designs in order to operate efficiently. Also, there is a drive to design smaller and more efficient electronic circuitry for many devices, such as PDAs, wireless telephones, cellular phones, portable computers, portable sensors and a variety of electronic equipment that generally require low power-consumption. The industry lacks an efficient way to improve the operation of refresh rates across many factors, such as temperature, voltage, and process variations.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.